If each us had a different kind of sense perception — if we could only perceive things now as a bird, now as a worm, now as a plant, or if one of us saw a stimulus as red, another as blue, while a third even heard the same stimulus as a sound — then no one would speak of such a regularity of nature, rather, nature would be grasped only as a creation which is subjective in the highest degree. After all, what is a law of nature as such for us? We are not acquainted with it in itself, but only with its effects, which means in its relation to other laws of nature — which, in turn, are known to us only as sums of relations. Therefore all these relations always refer again to others and are thoroughly incomprehensible to us in their essence.

On Truth and Lie in an Extra-Moral Sense (1873)